Introduction

For some use cases it is convenient and sometimes also mandatory to ensure that you have exactly one actor of a certain type running somewhere in the cluster.

Some examples:

single point of responsibility for certain cluster-wide consistent decisions, or coordination of actions across the cluster system

single entry point to an external system

single master, many workers

centralized naming service, or routing logic

Using a singleton should not be the first design choice. It has several drawbacks, such as single-point of bottleneck. Single-point of failure is also a relevant concern, but for some cases this feature takes care of that by making sure that another singleton instance will eventually be started.

The cluster singleton pattern is implemented by akka.cluster.singleton.ClusterSingletonManager. It manages one singleton actor instance among all cluster nodes or a group of nodes tagged with a specific role. ClusterSingletonManager is an actor that is supposed to be started as early as possible on all nodes, or all nodes with specified role, in the cluster. The actual singleton actor is started by the ClusterSingletonManager on the oldest node by creating a child actor from supplied Props. ClusterSingletonManager makes sure that at most one singleton instance is running at any point in time.

The singleton actor is always running on the oldest member with specified role. The oldest member is determined by akka.cluster.Member#isOlderThan. This can change when removing that member from the cluster. Be aware that there is a short time period when there is no active singleton during the hand-over process.

The cluster failure detector will notice when oldest node becomes unreachable due to things like JVM crash, hard shut down, or network failure. Then a new oldest node will take over and a new singleton actor is created. For these failure scenarios there will not be a graceful hand-over, but more than one active singletons is prevented by all reasonable means. Some corner cases are eventually resolved by configurable timeouts.

You can access the singleton actor by using the provided akka.cluster.singleton.ClusterSingletonProxy, which will route all messages to the current instance of the singleton. The proxy will keep track of the oldest node in the cluster and resolve the singleton’s ActorRef by explicitly sending the singleton’s actorSelection the akka.actor.Identify message and waiting for it to reply. This is performed periodically if the singleton doesn’t reply within a certain (configurable) time. Given the implementation, there might be periods of time during which the ActorRef is unavailable, e.g., when a node leaves the cluster. In these cases, the proxy will buffer the messages sent to the singleton and then deliver them when the singleton is finally available. If the buffer is full the ClusterSingletonProxy will drop old messages when new messages are sent via the proxy. The size of the buffer is configurable and it can be disabled by using a buffer size of 0.

It’s worth noting that messages can always be lost because of the distributed nature of these actors. As always, additional logic should be implemented in the singleton (acknowledgement) and in the client (retry) actors to ensure at-least-once message delivery.

Potential problems to be aware of

This pattern may seem to be very tempting to use at first, but it has several drawbacks, some of them are listed below:

the cluster singleton may quickly become a performance bottleneck,

you can not rely on the cluster singleton to be non-stop available — e.g. when the node on which the singleton has been running dies, it will take a few seconds for this to be noticed and the singleton be migrated to another node,

in the case of a network partition appearing in a Cluster that is using Automatic Downing (see docs for Auto Downing), it may happen that the isolated clusters each decide to spin up their own singleton, meaning that there might be multiple singletons running in the system, yet the Clusters have no way of finding out about them (because of the partition).

Especially the last point is something you should be aware of — in general when using the Cluster Singleton pattern you should take care of downing nodes yourself and not rely on the timing based auto-down feature.

Warning

Don’t use Cluster Singleton together with Automatic Downing, since it allows the cluster to split up into two separate clusters, which in turn will result in multiple Singletons being started, one in each separate cluster!

An Example

Assume that we need one single entry point to an external system. An actor that receives messages from a JMS queue with the strict requirement that only one JMS consumer must exist to make sure that the messages are processed in order. That is perhaps not how one would like to design things, but a typical real-world scenario when integrating with external systems.

Before explaining how to create a cluster singleton actor, let’s define message classes and their corresponding factory methods which will be used by the singleton.

Here we limit the singleton to nodes tagged with the "worker" role, but all nodes, independent of role, can be used by not specifying withRole.

We use an application specific terminationMessage(i.e. TestSingletonMessages.end() message) to be able to close the resources before actually stopping the singleton actor. Note that PoisonPill is a perfectly fine terminationMessage if you only need to stop the actor.

Here is how the singleton actor handles the terminationMessage in this example.

Configuration

The following configuration properties are read by the ClusterSingletonManagerSettings when created with a ActorSystem parameter. It is also possible to amend the ClusterSingletonManagerSettings or create it from another config section with the same layout as below. ClusterSingletonManagerSettings is a parameter to the ClusterSingletonManager.props factory method, i.e. each singleton can be configured with different settings if needed.

akka.cluster.singleton {
# The actor name of the child singleton actor.
singleton-name = "singleton"
# Singleton among the nodes tagged with specified role.
# If the role is not specified it's a singleton among all nodes in the cluster.
role = ""
# When a node is becoming oldest it sends hand-over request to previous oldest,
# that might be leaving the cluster. This is retried with this interval until
# the previous oldest confirms that the hand over has started or the previous
# oldest member is removed from the cluster (+ akka.cluster.down-removal-margin).
hand-over-retry-interval = 1s
# The number of retries are derived from hand-over-retry-interval and
# akka.cluster.down-removal-margin (or ClusterSingletonManagerSettings.removalMargin),
# but it will never be less than this property.
# After the hand over retries and it's still not able to exchange the hand over messages
# with the previous oldest it will restart itself by throwing ClusterSingletonManagerIsStuck,
# to start from a clean state. After that it will still not start the singleton instance
# until the previous oldest node has been removed from the cluster.
# On the other side, on the previous oldest node, the same number of retries - 3 are used
# and after that the singleton instance is stopped.
# For large clusters it might be necessary to increase this to avoid too early timeouts while
# gossip dissemination of the Leaving to Exiting phase occurs. For normal leaving scenarios
# it will not be a quicker hand over by reducing this value, but in extreme failure scenarios
# the recovery might be faster.
min-number-of-hand-over-retries = 15
}

The following configuration properties are read by the ClusterSingletonProxySettings when created with a ActorSystem parameter. It is also possible to amend the ClusterSingletonProxySettings or create it from another config section with the same layout as below. ClusterSingletonProxySettings is a parameter to the ClusterSingletonProxy.props factory method, i.e. each singleton proxy can be configured with different settings if needed.

akka.cluster.singleton-proxy {
# The actor name of the singleton actor that is started by the ClusterSingletonManager
singleton-name = ${akka.cluster.singleton.singleton-name}
# The role of the cluster nodes where the singleton can be deployed.
# If the role is not specified then any node will do.
role = ""
# Interval at which the proxy will try to resolve the singleton instance.
singleton-identification-interval = 1s
# If the location of the singleton is unknown the proxy will buffer this
# number of messages and deliver them when the singleton is identified.
# When the buffer is full old messages will be dropped when new messages are
# sent via the proxy.
# Use 0 to disable buffering, i.e. messages will be dropped immediately if
# the location of the singleton is unknown.
# Maximum allowed buffer size is 10000.
buffer-size = 1000
}

Supervision

There are two actors that could potentially be supervised. For the consumer singleton created above these would be:

Cluster singleton manager e.g. /user/consumer which runs on every node in the cluster

The user actor e.g. /user/consumer/singleton which the manager starts on the oldest node

The Cluster singleton manager actor should not have its supervision strategy changed as it should always be running. However it is sometimes useful to add supervision for the user actor. To accomplish this add a parent supervisor actor which will be used to create the ‘real’ singleton instance. Below is an example implementation (credit to this StackOverflow answer)